Update apt-get before installing dependencies (as recommended by travis:…

Update apt-get before installing dependencies (as recommended by travis: https://docs.travis-ci.com/user/installing-dependencies/#Installing-Ubuntu-packages)

.travis.yml

@@ -6,6 +6,7 @@ branches:
 python:
   - "2.7"
 install:
+  - "sudo apt-get update -qq"
   - "sudo apt-get install -y nodejs python-demjson"
   - "pip install --allow-all-external -r requirements.txt"
   - "pip install --allow-all-external demjson"